Numerous supports, such as plastics and glass, exhibit the disadvantage of becoming covered with fogging when the temperature of their surface falls below the dew point of the ambient air. This is the case in particular with glass, which is used to form windows for transportation vehicles or buildings, mirrors, ophthalmic lenses, such as spectacle lenses, and the like. The formation of fogging on these surfaces leads to a decrease in the transparency, due to the scattering of light by the water drops, which can cause significant nuisance.
In order to prevent the formation of fogging in a very humid environment, that is to say the condensation of minute water droplets on a support, it is possible to deposit, on the external surface of this support, hydrophilic coatings having a low static contact angle with water (typically 10° or less). These permanent antifogging coatings, the hydrophilic properties of which result from hydrophilic compounds permanently bonded to another coating or support, act as sponges with regard to the fogging and make it possible for the water droplets to adhere to the surface of the support by forming a very thin film which gives a sense of transparency. As a result of the absorption of water, they expand, soften and become mechanically weaker.
Another solution, which makes it possible to combine antireflective and antifogging properties, consists in using a thin porous layer having a low refractive index, partially composed of surfactants, which allow the layer to acquire permanent antifogging properties.
A more advantageous alternative to permanent antifogging coatings consists in obtaining an antifogging coating by application of a temporary hydrophilic solution to the surface of a precursor coating for an antifogging coating.
A description has been given, in international patent applications WO 2011/080472 and PCT/FR2012/051044, of the deposition, on the external surface of an optical article coated with a coating comprising silanol groups at its surface, of a hydrophilic precursor coating for an antifogging coating obtained by grafting a compound having a polyoxyalkylene group, in particular an organosilane. The antifogging coating proper, which is a temporary coating, is obtained after application, at the surface of the precursor coating, of a surfactant. Before this application of surfactant, the optical article may be edged (trimmed) satisfactorily if a temporary layer comprising at least 50% by weight of silica has been deposited at its surface. This silica-based temporary layer ensures the adhesion between the attachment member and the surface of the optical article, which makes it possible to hold it firmly during the trimming thereof while avoiding any slipping. It is removed at the end of the edging operation.
In these applications, the precursor coating for the antifogging coating typically has a thickness of less than or equal to 5 nm. This final thickness can be obtained by directly depositing the appropriate amount of organosilane compound.
However, the inventors have found that it is preferable to form the precursor coating for the antifogging coating by depositing an excess of hydrophilic compound at the surface of the coating of the lens, and by removing the excess of this compound, deposited but not grafted, so as to arrive at the same desired final thickness. This procedure guarantees the antifogging performance of the treated lenses. The inventors have indeed found that, when a layer of grafted hydrophilic compound having a thickness which does not require removal of an excess of hydrophilic compound is formed directly, it is possible, in some cases, to obtain an uneven deposited layer and a precursor coating for the antifogging coating, the surface of which does not have a sufficient affinity with regard to a surfactant liquid solution, which results in a coating that does not have the desired antifogging properties, and that is generally less durable. Such reproducibility problems absolutely have to be avoided in order for the production process to be able to be operated industrially.
Depositing a surplus of hydrophilic compound at the surface of the coating of the lens is one technical solution that itself also has drawbacks. The direct removal of the excess ungrafted hydrophilic compound, which is generally carried out by manual wiping, is long, difficult and therefore painful for the operator, which has a negative impact on the productivity, its cost and the motivation of the operator. Since the layer of hydrophilic compound is transparent, it is not possible to visually monitor its removal. Cosmetic problems linked to the difficult removal may also arise. The direct removal of the excess ungrafted hydrophilic compound is therefore one solution that it would be desired to avoid during the industrial manufacture of the lenses.